SAN FRANCISCO — Marijuana advocates are gathering signatures to get as many as three pot-legalization measures on the ballot in 2010 in California, setting up what could be a groundbreaking clash with the federal government over U.S. drug policy.

At least one poll shows voters would support lifting the pot prohibition, which would make the state of more than 38 million the first in the nation to legalize marijuana.

Such action would also send the state into a headlong conflict with the U.S. government while raising questions about how federal law enforcement could enforce its drug laws in the face of a massive government-sanctioned pot industry.

The state already has a thriving marijuana trade, thanks to a first-of-its-kind 1996 ballot measure that allowed people to smoke pot for medical purposes. But full legalization could turn medical marijuana dispensaries into all-purpose pot stores, and the open sale of joints could become commonplace on mom-and-pop liquor store counters in liberal locales like Oakland and Santa Cruz.

Under federal law, marijuana is illegal, period. After overseeing a series of raids that destroyed more than 300,000 marijuana plants in California’s Sierra Nevada foothills this summer, federal drug czar Gil Kerlikowske proclaimed, “Legalization is not in the president’s vocabulary, and it’s not in mine.”

The U.S. Supreme Court also has ruled that federal law enforcement agents have the right to crack down even on marijuana users and distributors who are in compliance with California’s medical marijuana law.

But some legal scholars and policy analysts say the government will not be able to require California to help in enforcing the federal marijuana ban if the state legalizes the drug.

Without assistance from the state’s legions of narcotics officers, they say, federal agents could do little to curb marijuana in California.

“Even though that federal ban is still in place and the federal government can enforce it, it doesn’t mean the states have to follow suit,” said Robert Mikos, a Vanderbilt University law professor who recently published a paper about the issue.

Nothing can stop federal anti-drug agents from making marijuana arrests, even if Californians legalize pot, he said. However, the U.S. government cannot pass a law requiring local and state police, sheriff’s departments or state narcotics enforcers to help.

That is significant, because nearly all arrests for marijuana crimes are made at the state level. Of more than 847,000 marijuana-related arrests in 2008, for example, just over 6,300 suspects were booked by federal law enforcement, or fewer than 1 percent.

State marijuana bans have allowed the U.S. Drug Enforcement Administration to focus on big cases, said Rosalie Pacula, director of drug policy research at the Rand Corp.

“It’s only something the feds are going to be concerned about if you’re growing tons of pot,” Pacula said. For anything less, she said, “they don’t have the resources to waste on it.”

In a typical recent prosecution, 29-year-old Luke Scarmazzo was sentenced to nearly 22 years and co-defendant Ricardo Ruiz Montes to 20 years in federal prison for drug trafficking through a medical marijuana dispensary in Modesto.

At his bond hearing, prosecutors showed a rap video in which Scarmazzo boasts about his successful marijuana business, taunts federal authorities and carries cardboard boxes filled with cash. The DEA said the pair made more than $4.5 million in marijuana sales in less than two years.

The DEA would not speculate on the effects of any decision by California to legalize pot. “Marijuana is illegal under federal law and DEA will continue to attack large-scale drug trafficking organizations at every level,” spokeswoman Dawn Dearden said.

The most conservative of the three ballot measures would only legalize possession of up to one ounce of pot for personal use by adults 21 and older – an amount that already under state law can only result at most in a $100 fine.

The proposal would also allow anyone to grow a plot of marijuana up to 5 feet-by-5 feet on their private property. The size, Pacula said, seems specifically designed to keep the total number of plants grown below 100, the threshold for DEA attention.

The greatest potential for conflict with the U.S. government would likely come from the provision that would give local governments the power to decide city-by-city whether to allow pot sales.

Hundreds of medical marijuana dispensaries across the state already operate openly with only modest federal interference. If recreational marijuana became legal, these businesses could operate without requiring their customers to qualify as patients.

Any business that grew bigger than the already typical storefront shops, however, would probably be too tempting a target for federal prosecution, experts said.

Even if Washington could no longer count on California to keep pot off its own streets, Congress or the Obama administration could try to coerce cooperation by withholding federal funds.

But with U.S. Attorney General Eric Holder’s announcement earlier this year that the Justice Department would defer to state laws on marijuana, the federal response to possible legalization remains unclear.

Doug Richardson, a spokesman for the White House’s Office of National Drug Control Policy, said the office is in the process of re-evaluating its policies on marijuana and other drugs.

Richardson said the office under Obama was pursuing a “more comprehensive” approach than the previous administration, with emphasis on prevention and treatment as well as law enforcement.

“We’re trying to base stuff on the facts, the evidence and the science,” he said, “not some particular prejudice somebody brings to the table.” Source.